Acetylcholine and histamine are known as vasoactive substances which may be involved in regulating cerebral blood flow (CBF) and/or blood-brain barrier (BBB) functions. This supposition has been supported by detection of cholinergic and histaminergic fibers in the close vicinity of cerebral vessels. Moreover, receptors for both substances were described in microvessels isolated from brain of animals but not from man. The aim of this investigation was to examine the effect of acetylcholine, carbachol and histamine on adenylate cyclase (AC) activity in the endothelium derived from three fractions of human microvessels. The response of adenylate cyclase to acetylcholine, carbachol and histamine was seen in all examined homogenates of endothelium (4th-11th passage) derived from large microvessels. Pirenzepine (M1-muscarinic inhibitor) blocked the acetylcholine and carbachol stimulated formation of cAMP while methoctramine (M2 muscarinic inhibitor) had no effect on AC of endothelium derived from large microvessels. Preliminary investigations demonstrated a dose-dependent cholinergic stimulation of cAMP production in endothelium derived from capillaries and small microvessels. The response of endothelial AC activity to histamine was inhibited (30-100%) by cimetidine indicating the AC linkage to H2-receptors. These findings represent the first demonstration of cholinergic and histaminergic receptors linked to the AC system in cerebromicrovascular endothelium of human brain. The loss of endothelial cultures due to toxic human serum prevented the completion of this project.